In prior approaches, the arbitration mechanism is often centralized and cannot be distributed physically or into different clock cycles. If pipeline points separate multiple arbitration points, the arbitration policy is often dependent on the physical distribution of the arbitration points. This makes it difficult to change the physical distribution, such as might happen during the late stages of chip physical layout, without changing the logic implementing the arbitration policy.
Adding quality-of-service features and/or multi-threading can complicate the arbitration mechanism and can make it much slower.